Please tell me they are common, was planning on making it my next sniper rifle but accidently sold it :/

You can buy plasma pistol in diamon city and mod it to rifle

Nice nice ty

Who's selling it?

i can find it in the middle guy's shop from time to time. especially after level 15. but he doesn't have it instock all the time.

level up a bit and they're all over the place, you need to mod em for the most part but i keep the short barrel version and it works great , the diamond city or good neighbor vendors have plenty of weapons.

You could have just bought it back